Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="background"></div>

SassMeister-input.scss

// ----
// Sass (v3.3.9)
// Compass (v1.0.0.alpha.20)
// ----

@mixin ratio($width, $height) {
  
  &:before {
    content: '';
    display: block;
    padding-bottom: percentage($height / $width);
  }
}

.background {
  @include ratio(25,10);
  max-width: 50%;
  margin: 0 auto;
  
	&:before {
    background: red;
  }
}

SassMeister-output.css

.background {
  max-width: 50%;
  margin: 0 auto;
}
.background:before {
  content: '';
  display: block;
  padding-bottom: 40%;
}
.background:before {
  background: red;
}

SassMeister-rendered.html

<div class="background"></div>